The present invention relates to an information recording medium and a method for reproducing the same, and, more particularly, to an information recording medium suitable for use in an optical disk file of the sector servo type and a method for reproducing the read out clock of the same.
In an optical disk file system, for a user to record information, there has been adopted a method by which a pre-groove is formed in advance in a disk so that data pits may be recorded in said pre-groove while tracking with reference to the pre-groove. An example of such a pre-groove type optical disk file system is disclosed in detail in Nikkei Electronics (pp. 189 to 213), on Nov. 21, 1983.
In recent years, on the contrary, there has been proposed a sector servo type optical disk file system. According to this file system, one track on the disk is divided into small fields called "sectors", each of which is formed at its leading end with a servo field for generating control information such as tracking information so that a data recording area formed at the trailing end of the servo field may be recorded with data by controlling the tracking servo on the basis of the tracking information obtained from the servo field. Therefore, the sector servo type optical disk does not use any pre-groove, but obtains the tracking information only at the leading end of each sector so that the tracking control is a sampling type of control. In the pre-groove type, on the contrary, the tracking information is always obtained from the pre-grooves arranged continuously so that the tracking control is a continuous type of control. The sector servo type optical disk file system is disclosed in, for example, "Sector Servo Optical Disk File System," preprints of 45th Seminar of Japanese Association of Applied Physics (in October, 1984), pp. 56, 13p-E-8 and 13p-E-9. Incidentally, the sector servo type, has been proposed long before in the field of the magnetic disk, as in U.S. Pat. No. 3,185,972.
The sector servo type optical disk file system is preferred over the pre-groove type in that the tracking is more stable even with an optical pickup of simple construction.
On the other hand, the sector servo type requires a system which is different from that of the pre-groove type for writing and reading the data. In the sector servo type optical disk, the number of sectors for one complete loop of the disk (hereinafter referred to as a track) has a close relation to the tracking control and needs to be about 500.about.1000 sectors/track. It is, however, generally 100 sectors/track or less for the sector servo type magnetic disk. This value results from the fact that the optical disk has a tracking pitch of about 1.6 .mu.m whereas the magnetic disk has a tracking pitch of 50 to 100 .mu.m so that the latter disk has an easier tracking control. In the pre-groove type disk of the prior art, one track of the disk is divided into 100 sectors/track or less, and this sector number has no significant meaning in the tracking control, but is determined merely by a factor of the magnitude of the data processing unit.
FIG. 1 is a schematic view showing the shape of an optical disk 1, in which tracks 2 are arranged at a pitch of about 1.6 .mu.m and each is divided into a plurality of sectors 3. The sector number of one track is 100 or less, as mentioned above, in the pre-groove type of the prior art and about 500 to 1,000 in the sector servo type.
FIG. 2 schematically shows the structure of one sector in the pre-groove type optical disk. If an optical disk having a diameter of 30 cm has a sector number of 64 sectors/track, for example, the length of each sector 3 corresponds to 690 bytes. Each sector 3 is divided into a pre-formatted area 31, which has been recorded in advance when the disk is fabricated, and a data recording area 32 for the user to record the data. The former pre-formatted area 31 is composed of: a sector mark SM for indicating the leading end of each sector; an identification signal (which will be shortly referred to as "ID") for indicating a track address and a sector address; and a synchronization signal SYNC A used for synchronizing a read out clock for reading out the ID. The method for reading out the ID information and data written in the sectors thus formatted according to the prior art will be described with reference to FIG. 3. FIG. 3(a) is a block diagram showing a signal read out system, in which the information on the disk is photoelectrically converted into an electric signal by an optical beam incident upon a photo detector 4 in the optical head of the optical disk system to generate a signal 51 amplified by a pre-amplifier 5. For simplicity, the signal 51 in FIG. 3 is digitized by a suitable processing. Here, it is assumed that a focusing control and a tracking control are performed so that the optical spot traces the tracks. The sector mark indicating the leading end of each track is recorded in a special pit pattern different from another signal. When it is detected by a sector mark detector 6 that the special sector mark pattern has been read out, the processing system is informed of the fact that the optical spot leads the sector and that the signal to be subsequently input is the SYNC A signal, followed by the ID signal. On the other hand, the output 51 from the pre-amplifier 5 also is fed to a phase locked loop circuit 7 (which hereinafter will be referred to as "PLL") so that this circuit 7 generates a read out clock 71 for reading out the ID signal by using the SYNC A signal accompanying the sector mark. Moreover, a timing detector 8 also outputs a signal 81 accurately indicating the timing for starting the ID signal by using the SYNC A signal. As a result, with reference to the read out clock 71 generated by the PLL circuit 7 and the timing pulse 81 generated by the timing detector 8, a decoder 9 reads out the ID signal, i.e., the track address and the sector address.
In case the user data is to be written in the data recording area 32 of each sector, the ID information is read out, as above, to identify the target sector, and the data is then recorded in the data recording area of that sector. The information to be written in the data recording area requires not only the user data (DATA) but also a preceding data SYNC B. This SYNC B signal is used like the SYNC A signal of the pre-formatted area to generate the read out clock and timing for reading out the DATA. Incidentally, since the information of the pre-formatted area 31 and the information to be written in the data recording area have different timings in the writing thereof, the pits of information to be written in naturally are out of phase. This makes it necessary to generate the independent synchronization signals as two separate areas on the dish. FIG. 3(b) is a block diagram showing the structure of the PLL circuit 7. This PLL circuit is roughly divided into three parts, i.e., a phase detector 701, a low-pass filter 702, and a voltage controlled oscillator (which hereinafter will be referred to as "VCO") 703 to provide a mechanism for generating the read out clock 71 in phase with the input signal 51. Incidentally, in the read-out of either the ID information or the user data, the PLL circuit is operating to correct the phase of the read out clock by using the ID and DATA signals themselves being read out even after it has had its phase corrected with the synchronization signals SYNC A and SYNC B. As a result, the read out clock does not come out of phase even in the case of an enlarged DATA length so that the correct read-out can be conducted. Incidentally, the function to correct the read out clock by using the signal itself is called "self-clocking".
The description thus far is directed to the signal reproducing method of the pre-groove type of the prior art. FIG. 4 shows sector servo type sectors 3, each of which is divided into a servo field 33 and a data recording area 34. Because of shortage of the pre-groove in the present system, the tracking is conducted by using the tracking information stored intermittently in advance in the servo field 33 of each sector. As a result, the sector number per one track increases to 500 to 1,000, i.e., 10 to 100 times as large as that of the pre-groove type. For example, the pre-groove type shown in FIG. 2 has a sector number of 64 per one track and a one sector length of about 700 bytes, of which a length of about 35 bytes is occupied by the pre-formatted area 31 whereas the synchronization signals SYNC A and SYNC B have a length of about 10 bytes. In the case of the sector servo type of FIG. 4, on the contrary, for example, the sector number is about 1,000, and the sector length is about 45 bytes, of which 2 to 4 bytes are occupied by the servo field. As is apparent from those numerical values, the sector of the sector servo type is far shorter than that of the pre-groove type. Here, if the sector is made to have a structure similar to that of the pre-groove type of FIG. 2, a length of 35 bytes of the one sector length of 45 bytes is required for the synchronization signals and the ID so that the data recording area is remarkably reduced to a value failing to satisfy the practical requirement. Moreover, the length of 35 bytes for the synchronization signals and the ID information is difficult to compress in one sector. It is therefore practically impossible for the optical disk of the sector servo type to incorporate the ID information and the synchronization signals for the phase registration into each of the sectors, thus making it necessary to provide a novel sector format and a novel method for reading out the sector format.